In U.S. Pat. No. 5,374,989 an apparatus for identifying an object having a non-specific outer boundary is disclosed. The apparatus comprises a tracking device for tracking a position of the object to be identified in two dimensions and an identification device for identification of the tracked object. The objects to be identified may be stained so as to distinguish them from other objects by a difference in transmissivity at a specific range of wavelengths. The object identification device comprises a coherent light source illuminating an area of the specimen and a detection device with a plurality of circular coaxial sections which are detecting the intensity distribution of a Fourier transform of light transmitted through the objects of the specimen.
U.S. Pat. No. 5,663,057 discloses an apparatus and a process for rapid counting of fluorescent microorganisms in a specimen. A solid support, such as a filter, holding the fluorescent microorganisms is scanned line by line with a laser beam having a circular focus area, with a diameter between 4 and 14 μm, on the filter. The emitted fluorescence is measured at one or more predetermined wavelengths. The scanning lines on the filter are produced in an overlapping pattern, and line-to line features are compared in time synchrony to eliminate uncorrelated events like random noise etc. A digital signal processing circuit applies a set of predetermined mathematical algorithms to the obtained fluorescent light signals to avoid or minimise detection of false positive responses and false negative responses.
The above mentioned types of prior art apparatuses utilises a Cartesian (x,y) scanning and tracking methodology, whereby the specimen is scanned line by line and the particular type of objects are recognised by either detecting emitted light from the objects or by detecting characteristic parameters of an optical Fourier transformation of light transmitted through the object.
These prior art methods of scanning specimens to locate the objects are too slow for practical purposes when the particular type of objects or target objects are very rare in the specimen, i.e. they may have a density of 1E-6 or less. The slowly scanning is due to the Cartesian scanning of the specimen and the small focus area of the utilised laser beams combined with the large number of objects, which have to be analysed before it is likely that a target object is located and identified. Further, due to the limited dimensions of the specimen provided by these prior art methods, it may not contain any target objects at all.